More than half-a-million Americans suffer strokes every year, and a similar number suffer head trauma each year. Some of these people die of their injuries; however, most survive with some degree of neurological damage. Although many treatments and therapies have been attempted, none are very effective in reducing neurological damage following stroke, head trauma, or other such condition. One of the most effective therapies known for these and similar conditions is hypothermia, the lowering of body temperature. Even small reductions in body temperature after the initial injury has occurred can reduce damage and improve neurological outcome, if treatment is not delayed too long after the incident. In fact, hypothermia has been a confounding variable in animal experiments directed at discovering pharmacological compounds which may reduce neurological damage following experimentally-induced trauma; some compounds thought to be directly neuroprotective have instead been found to lower body temperature in the small-bodied experimental animals used in studies. When the body temperature of these animals is artificially maintained at the normal level during drug treatment, some of these promising drugs have been found not to be neuroprotective, revealing that the effective treatment was hypothermia, and not the drug. Accordingly, it is desireable to discover a method for inducing hypothermia in humans.
Body temperature is very well regulated in warm-blooded animals. However, the pharmacological compounds that may lower the body temperature of a mongolian gerbil or a rat (with large body surface areas compared to their small volumes) do not effectively produce hypothermia in larger animals such as humans. It is very difficult to cool humans due to our larger mass, smaller surface area in proportion to our volume, and our complex homeostatic mechanisms geared towards maintaining our body temperature.
However, although difficult, it is possible to cool large animals and humans. Hypothermia (the condition of lower-than-normal body temperature in a warm-blooded animal) has been investigated in animals for many years, and has been used on human patients (for example, in heart surgery) for more than forty years. It is known to reduce neurological damage otherwise resulting from cardiac arrest, stroke and trauma. Known methods for inducing hypothermia all involve cooling the outside or inside of an animal, sometimes in conjuction with drugs that disable the animal's homeostatic responses. Present methods for inducing hypothermia include externally applied cold packs, ice blankets, infusion of cold saline into arteries and into the peritoneum of an animal, blowing air across an animal's skin, wetting the skin or hair of an animal, and cooling the air around an animal. One method, preferred by some researchers, includes infusion of saline into the peritoneum of an animal in order to cool a large volume of blood and tissue in contact with the peritoneum. However, this is no simple procedure, but is an invasive procedure that requires puncture of the abdominal wall, infusion of cool or cold saline, and monitoring of fluid and electrolyte balance of the animal for the duration (and beyond) of the procedure. Hypothermia may also be a side-effect of general anesthesia during surgery.
However, these methods are impractical because they require trained personnel and dedicated equipment, and often induce discomfort in the animal or are invasive. In many of the above examples, careful co-ordination and oversight by medical personnel is required since drastic measures need be taken to overcome the normal operation of the animal's physiological responses to cold. These responses include vasoconstriction, shunting of blood away from the limbs and retention of blood in the body core (away from cold blankets, wet skin, etc.) and shivering. Suppression of these responses by muscle relaxants, vasodilators and other drugs may also cause, as side-effects, suppression of other vital body functions associated with breathing, maintenance of blood pressure, heart rate, and other vital bodily functions. These side-effects, such as circulatory shock, may be serious. They increase risk and limit the effectiveness of hypothermia treatments in humans. Thus, there is at present no simple, effective method for inducing and maintaining hypothermia in an animal.
All of the above-mentioned methods for inducing hypothermia, with the possible exception of some potent centrally-active drug regimens that disable thermoregulation, must work to oppose the animal's bodily efforts to maintain body temperature. Difficulties with these techniques arise because the homeostatic mechanisms and physiological responses involved in regulation of body temperature are among the most basic responses in warm-blooded animals. Cooling by cold blankets and dressings is uncomfortable, induces shivering which must be opposed by medication, and causes vasoconstriction which reduces blood flow to the cooled extremities, reducing the effectiveness of the cooling treatment. Cooling by intra-arterial infusion of cold blood or saline is invasive, of limited utility because of limited ability to deliver large volumes of cooled fluid, requires medical equipment and supervision, and may potentially cause vascular, cardiac and neurological (if emoboli are created) side-effects. Pharmaceutical treatments that disable thermoregulatory responses often have other effects as well, and require active cooling measures such as those already mentioned in order to lower body temperature of a large animal. Thus, an ideal method for inducing hypothermia in a warm-blooded animal would not require drastic invasive measures or drugs, and would not oppose the animal's physiological temperature control mechanisms, but would make use of them to achieve hypothermia.